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Ozone addition is not only a promising method for combustion control and enhancement but also provides a reliable platform to benchmark low-temperature oxidation chemistry. Studies of ozone-doped combustion provide additional insight into combustion chemistry at lower temperatures. This paper studies the ozone-assisted low-temperature oxidation of propane in an atmospheric jet-stirred reactor (JSR) from 350 to 770 K. More than twenty species are measured and quantified using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). The experimental results show that even at temperatures as low as 450 K, the reaction of propane is initiated by Ö atoms that originate from the thermal decomposition of ozone. The reactivity of propane reaches a maximum at 650 K, decreases in the temperature range of 650 – 740 K, and then slightly increases from 740 to 770 K. The propane oxidation behavior observed in this study is effectively captured by NUIGMech1.3 combined with a Princeton ozone sub-mechanism. However, the mole fractions of certain species such as, CO, H 2 O, and H 2 O 2 are over-estimated by the mechanism at temperatures above 700 K. Kinetics analyses indicate that fuel oxidation and intermediate species formation are sensitive to the reaction temperature which determines the competing reactions involving propyl-peroxy radicals and the formation of ȮH and HȮ 2 radicals. These findings are significant in terms of improving the core mechanism and acquiring a deeper comprehension of the ozone-assisted low-temperature oxidation chemistry of alkanes. [ABSTRACT FROM AUTHOR]

Oxidation of pure ammonia or ammonia/dimethyl ether (NH 3 /DME) mixtures in O 2 /Ar in a jet-stirred reactor (JSR) at atmospheric pressure was experimentally studied using a synchrotron vacuum ultraviolet photoionization mass spectrometer (SVUV-PIMS) and a gas chromatograph (GC) to analyse the reaction intermediates and product species. Particular attention was given to the effect of DME addition on NH 3 → NH 2 , a critical step that initiates NH 3 oxidation and subsequent formation of nitrogen oxides. The initial mole fraction of NH 3 was set to 0.01, and for the NH 3 /DME mixture, the NH 3 /DME molar ratio was set to 1. The experiments were performed at two equivalence ratios of ϕ = 1.0 and 0.2 in the temperature range of 500 – 1100 K. A kinetic model for NH 3 /DME oxidation was developed and kinetic analysis was performed. It was found that the addition of DME significantly promotes the NH 3 to NH 2 pathway and the mechanisms of this promotion effect are different at low and high temperatures. At a low temperature (∼ 600 K), NH 3 → NH 2 is mainly promoted by OH radicals produced from the DME low-temperature chain-branching reactions, while at a high temperature (∼ 1000 K), mainly by OH radicals produced from the reactions consuming HO 2 and H radicals, which were formed from DME high temperature oxidation. At T > 800 K with ϕ = 0.2, the addition of DME promotes the formation of nitrogen oxides, however, at a low temperature of 600 K, the addition of DME only promotes the formation of N 2 O and NO 2 but not NO. A reaction pathway analysis indicates that the oxidation of NO into NO 2 at low temperature is promoted by HO 2 and CH 3 OCH 2 O 2 , the intermediate products from the low temperature oxidation of DME. [ABSTRACT FROM AUTHOR]

n -Dodecane is a potential diesel and jet fuel surrogate. The combustion chemistry of n -dodecane has long been a research focal point, but its long carbon-chain length makes it challenging to obtain detailed information on cool flame intermediates and products. Determining the detailed formation mechanism of combustion intermediates is important to validate possible reaction pathways, develop detailed reaction mechanisms, and improve kinetic models. Reports on its low-temperature (low-T) oxidation, including detailed speciation in the oxidation process, are scarce. In this work, low-T oxidation of n -dodecane was investigated in a jet-stirred reactor coupled with synchrotron vacuum ultraviolet photoionization mass spectrometry and gas chromatography. Experimental conditions were maintained at atmospheric pressure with an initial fuel concentration of 0.4%, equivalence ratio of 0.5, and residence time of 2 s. A large number of cool flame intermediates and products, including alkenes, dienes, aldehyde/keto compounds, olefinic aldehyde/keto compounds, diones, cyclic ethers, peroxides, acids, and alcohols/ethers, were identified and classified. Particular attention is focused on understanding these intermediates and products. Detailed structures and the most likely formation channels for these species are proposed and discussed herein. These results provide a solid database for the low-T oxidation of n -dodecane, which is important for the development of future kinetic models. [ABSTRACT FROM AUTHOR]

Ammonia is a promising carbon-free fuel and has attracted massive attention nowadays. However, there are still large uncertainties in the mechanism of ammonia oxidation that motivate further fundamental investigations. In this work, oxidation experiments of ammonia were carried out in a jet-stirred reactor (JSR) equipped with synchrotron vacuum ultraviolet (SVUV) photoionization mass spectrometry and gas chromatography. Ammonia/O 2 /Ar mixtures were tested under wide-range operating conditions covering an oxidation temperature of 700–1200 K and an equivalence ratio of ϕ = 0.1–1.0. Important intermediates and products (e.g., NO, NO 2 , N 2 O, N 2 , H 2 , and H 2 O) were detected and quantified. A chemical kinetic model for ammonia oxidation was proposed by integrating the rate constants of critical reactions from different models. The results show that ammonia oxidation reactivity is quite strong at ultra-lean conditions, manifesting lower oxidization temperatures and greater oxidization rates. However, this behavior becomes weakened rapidly with the elevation of equivalence ratio, such that ammonia oxidation characteristics are highly similar between ϕ = 0.5 and 1.0. Meanwhile, important intermediates like NO 2 and N 2 O are only detected under ultra-lean conditions. Reaction pathway analysis for equivalence ratios shows that high oxygen concentration is beneficial to the formation of HO 2 , which leads to a huge amount of OH and promotes the dehydrogenation of ammonia. With the elevation of equivalence ratio, the dehydrogenation of ammonia becomes difficult and reactions NO+HO 2 =NO 2 +OH and NH 2 +NO 2 =N 2 O+H 2 O are suppressed, which answers for the vanishing of NO 2 and N 2 O at ϕ = 0.5 and 1.0. Reaction pathway analysis for oxidation temperatures also emphasizes the significance of HO 2 and its conversion pathway to OH, but they are also strongly influenced by equivalence ratios. The current work provides useful insights into ammonia oxidation and a reliable experimental database for modeling. [ABSTRACT FROM AUTHOR]

Comprehensive analysis of the low-temperature oxidation intermediates, including the reactive intermediates and isomers, is crucial to develop the low-temperature oxidation models of the fuels. In this aim, the complementary analysis by different analytic methods is needed. Furthermore, the cross check by different analytic methods increases the fidelity of the experiment data. In this work, we developed a jet-stirred reactor (JSR) system that coupled to the synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and gas chromatography (GC) analysis. The JSR was designed to couple the SVUV-PIMS and GC, and the temperature homogeneity and mixing of the JSR was examined by a CFD simulation. The SVUV-PIMS system was developed to have a mass resolution of ca. 5000 at m/z 100. The GC system included a flame ionization detector (FID), thermal conductivity detector (TCD), and a mass spectrometer (MS). This newly developed setup was validated by repeating the oxidation data of n -heptane in the literature. A good agreement was observed between the two datasets. The low-temperature oxidation of n -butane was then studied to show the advantage of this setup, i.e., the simultaneous and comprehensive measurement of the low-temperature oxidation products. Specifically, the SVUV-PIMS enables the separation of the C/H/O composition of the intermediates, the measurement of formaldehyde and the peroxide intermediate, while the GC analysis gives a better separation of the oxygenated isomers and the quantification of the species that the SVUV-PIMS has difficult in quantifying. The experimental method developed in this work is valuable to study the fuel low-temperature oxidation chemistry, to reveal the species pool, and to provide reliable mole fraction data for the validation and development of combustion models. [ABSTRACT FROM AUTHOR]

N-methyl pyrrole (NMP) is an important nitrogenous heterocyclic compound in bio-oils produced from biomass conversion. NMP can be used as surrogate model compound to clarify the combustion chemistry of nitrogenous fuels and NO X formation mechanism. The low- to moderate-temperature oxidation experiments of NMP are carried out in atmospheric pressure jet-stirred reactor for 0.5% NMP/oxygen/Argon mixtures at different equivalence ratios (0.5, 1.0, and 2.0) over the temperature range of 760 – 1030 K. More than twenty species, including reactants, nitrogen-containing species, and oxygenated products, are measured using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). A detailed and comprehensive kinetic model for NMP oxidation is developed for the first time based on the pyrrole oxidation model developed by Pelucchi et al. and Chen et al.. Simulated results show that the present model can reasonably reproduce the experimental results. Rate of production analysis indicates that the H-abstraction reactions on the methyl group dominate in NMP oxidation at all equivalence ratios. NMP oxidation produces C4 and C5 nitrogen-containing compounds including pyridine and pyrrole. These species are found to be products of the primary reactions of NMP, and their formation pathways can be explained satisfactorily by the present model. The present work provides a basis to further understand the oxidation characteristics of nitrogen-containing heterocyclic fuels. [ABSTRACT FROM AUTHOR]

Pyridine is an important model compound of fuel-nitrogen during the coal combustion process. The lean oxidation of pyridine was studied over 800–1100 K at atmospheric pressure with the equivalence ratio of 0.4 in a jet-stirred reactor. Pyridine and its oxidation products were measured by tunable synchrotron vacuum ultraviolet photoionization molecular-beam mass spectrometry and gas chromatography analyzer. Three-stage consumption phenomenon of pyridine and O 2 was observed including a fast consumption stage at 910–920 K, a moderate consumption stage at 920–1000 K, and a slow consumption stage at 1000–1100 K. HCN, N 2, N 2 O, and HNCO are the major nitrogenous oxidation products of pyridine. CO, CO 2 , HCN, and HNCO are the dominant carbonaceous products. HNCO is uniquely detected in the lean oxidation of pyridine compared with rich oxidation. A new pyridine LTO 2.0 model comprising 210 species and 1349 reactions was proposed, providing a good prediction of pyridine and its major oxidation products against experimental data. N 2 O emission is larger than NO at 1100 K. HCN is the dominant nitrogenous intermediate under both fuel-lean and fuel-rich conditions. Results of this work provide a detailed analysis of the nitrogen evolution mechanism of pyridine oxidation under the fuel-lean condition, which is close to the real industrial low-temperature coal combustion processes including the MILD combustion, fluidized bed combustion, etc. [ABSTRACT FROM AUTHOR]

Conformational effect is an important feature of the low-temperature (low-T) oxidation chemistry of cycloalkanes. In this work, both theoretical calculation and conformational analysis were performed for the first oxygen addition in methylcyclohexane (MCH) oxidation to better understand the influence of the spatial orientation and the steric hindrance of substituted functional groups. Based on synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) measurements and adiabatic ionization energy calculations, reactive hydroperoxides, highly oxygenated molecules and stable products were identified in the jet-stirred reactor (JSR) oxidation of 1.0% and 0.5% MCH at 1.04 bar, 500–900 K and the equivalence ratio of 0.25. The abundant formation of 1-methylcyclohexene and the observation of alkenylhydroperoxides and their decomposition products verify the existence of conformation-dependent pathways in first and second oxygen addition in low-T MCH oxidation, respectively. A MCH oxidation model incorporated with conformation-dependent pathways was constructed to consider the conformational effects of cyclohexane ring and sidechain, and was validated against experimental data over a wide range of conditions in this work and literature. The consideration of conformational effects of cyclohexane ring and sidechain can generally better predict the measured species profiles and ignition delay time under low-T conditions. Based on modeling analysis, conformational effects of cyclohexane ring were found to play important roles in first oxygen addition and chain-branching in low-T MCH oxidation. Compared with the situation in low-T cyclohexane oxidation, the methyl substitution on cyclohexane ring reduces its conformational effects on chain-branching process. [ABSTRACT FROM AUTHOR]

Benefitting from the rapid development of instrumental analysis methods, intermediate products that were difficult to probe in the past can now be measured and quantified in complex reaction systems. To understand low temperature reactions of interest for combustion applications, and reduce the deviations between model predictions and experimental measurements, constant advancement in understanding low temperature oxidation process is necessary. This work examines the oxidation of n -heptane in jet-stirred reactors at atmospheric pressure, with an initial n -heptane mole fraction of 0.005, equivalence ratio of 0.5, a residence time of 1s, and over a temperature range of 500-800 K. Reaction products were analyzed using synchrotron ultra-violet photoionization mass spectrometry, gas chromatography, and Fourier-transform infrared spectroscopy. Ignition delay times of n -heptane/O 2 /CO 2 mixture were measured in a rapid compression machine at 20 and 40 bar over a 600-673 K temperature range. Based on the experimental results, a comprehensive kinetic model of n -heptane low temperature oxidation was developed by considering the sub-mechanisms of keto-hydroperoxide, cyclic ether, heptene isomers, and the third O 2 addition reaction, and by updating the rate constants of keto-hydroperoxide decomposition and second oxygen addition reactions. The combination of reaction mechanism development and evaluation of the rate constants of key reactions enabled the model to effectively predict the species concentrations and ignition delay times of n -heptane low temperature oxidation, providing additional insight into alkane low temperature oxidation chemistry. [ABSTRACT FROM AUTHOR]

This paper explores the low-temperature (low-T) oxidation chemistry of 2,4,4-trimethyl-1-pentene (IC8D4, diisobutylene) by using jet-stirred reactor (JSR) experiments of both IC8D4/dimethyl ether (DME) mixture and pure IC8D4 at near atmospheric pressure and low temperatures. Oxidation species are measured using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS), gas chromatography (GC) and GC combined with mass spectrometry (GC/MS). It is found that the oxidation of pure IC8D4 at atmospheric pressure presents negligible low-T reactivity and negative temperature coefficient (NTC) behavior, and that the oxidation reactivity of IC8D4/DME mixture is lower than that of butene/DME mixtures previously studied. A kinetic model for low-T IC8D4/DME oxidation is developed from recent oxidation models of IC8D4 and DME. Thermodynamic data of IC8D4 and key species in its sub-mechanism are obtained from theoretical calculations in this work, while rate constants of critical reactions are updated from recent theoretical calculation studies in literature. Based on the modeling analysis, four main pathways are found to be responsible for the consumption of IC8D4 at low temperatures. Among them, the first three pathways initiated by H addition, OH addition and H abstraction on allylic carbon sites are similar to those in 1-butene/DME and isobutene/DME oxidation. These three pathways are mainly responsible for promoting or retaining OH formation. The fourth pathway initiated by H abstraction on the alkyl carbon site differs from the other three in that it is only important in IC8D4/DME oxidation while not important in butenes/DME oxidation. This fourth pathway incorporates stepwise O 2 addition and cycloaddition reaction sequences, which can promote and inhibit OH formation, respectively. The increasing contribution of this fourth pathway in IC8D4/DME oxidation reduces its reactivity compared to that of butenes/DME oxidation. [ABSTRACT FROM AUTHOR]